The size 24 will be 2mm narrower in the forefoot than the 25. That's the industry standard. It will be narrower in the heel area also but I don't know if it will be 2mm narrower. The Kryzma is a 98mm lasted boot, but the 98mm is based on a size 24 boot so your size 25 is actually 100mm. Are your current boots actually marked 25 or 25.5. Half sizes in ski boots don't exist, except for the boxes the boots come in. If you have a narrow low volume foot, I would recommend you look at some other brands. Lange makes some low volume boots, Nordica is coming out with one this next season, the Atomic Redster Pro 90W is another as are the Head Raptor 110W and 90W. If you like the three piece design, Full Tilt is a narrower lower volume boot than the Dalbello and uses Intuition liners.
If you heel is really loose in your current boot, going down one size is unlikely to correct that. A good fitter can add some padding to the outside of the liner before cooking it, but that is not the best way to fix the problem. The best was is to just find a boot with the right size heel pocket.